1. Field of the Invention
The invention relates to a process for producing a ceramic evaporation boats that have an improved initial wetting performance.
2. Description of the Prior Art
The most widely used method of coating flexible substrates with metals, in particular with aluminum is high vacuum tape coating. The substrate to be coated is passed over a cooled roller while being exposed to aluminum vapor which deposits on the substrate surface as a thin metal layer.
To generate the constant vapor stream required, ceramic evaporators known as evaporation boats, are heated to about 1450xc2x0 C. by a direct passage of electric current through the evaporation boat. Aluminum wire is continuously fed to the boat, liquified on the ceramic surface and evaporated in a vacuum of about 10-4 mbar. In metallization units, a series of evaporation boats are arranged in such a away that a uniformly thick aluminum layer is deposited across the entire width of the substrate.
Evaporation boats are generally made of hot-pressed titanium diboride (TiB2) and boron nitride (BN) and/or aluminum nitride (AlN). In these evaporation boats, TiB2 is the electrically conductive component which allows the evaporator to be heated like an ohmic resistance.
One of the main problems in the operation of web coating units is the initial wetting of the evaporation boats with the metal to be vapor-deposited. In practice, the operator has to have a great deal of experience to be able to carry out the initial wetting of the evaporation boats in an optimal way. Thus, the term xe2x80x9cbreak-in procedurexe2x80x9d has become established to describe this initial wetting of the evaporation boat which illustrates the complexity of this step. Thus, during the running up phase, the cavity of the evaporator can be incompletely wetted. This results in increased deposits on the side opposite that where the wire is fed which forces the operator to run the evaporation boat xe2x80x9chotxe2x80x9d for a given evaporation rate, i.e. to heat it to a very high temperature. This leads to a drastic decrease in the life of the evaporation boats.
In addition, incomplete wetting corresponds to non-uniform wetting of the cavity of the evaporation boat. As a result, uniform, continuous evaporation of the metal to be evaporated is not possible later. This forces the operator to adjust the evaporator heating continually. As a result, the evaporation boats, on average, run too hot. This result greatly reduces the life of the evaporation boats, as already mentioned.
In the case of evaporation boats having a very high electric resistance, the voltage of the vapor deposition unit is generally insufficient to heat it to the wetting temperature. If such an evaporation boat were to be wetted more readily than conventional evaporation boats, some wetting would take place even before the full wetting temperature reached. As a result, the system evaporation boat-aluminum bath has a lower electric resistance. This immediately produces a higher current which in turn leads to better heating of the evaporation boat and consequently also to even better wetting.
This feature also makes the problem of the difference in the resistance from evaporation boat to evaporation boat significantly less critical and stoppage of the vapor deposition units due to high-resistance evaporation boats does not occur.
The better wetting of the evaporation material by the metal to be evaporated, the lower the risk of the evaporation boat being overheated and thus the life of the evaporation boat being drastically reduced. In addition, good wetting leads to optimum metal bath formation in the cavity of the evaporation boat and thus to improved evaporation conditions and more uniform stressing of the evaporation boat which, in turn, increases the life of the evaporation boat.
It is an object of the invention to provide an evaporation boat of ceramic material for the evaporation of metal comprising a conductive component and a nonconductive component, wherein the boat is initially wetted more readily by the metal to be evaporated.
The object is achieved by providing an evaporation boat wherein the conductive component of the ceramic material is concentrated at the surface of the evaporation boat at which the evaporation of the metal occurs.
Preferably, the content of the conductive component at the surface of the evaporation boat at which evaporation of the metal occurs is at least two wt % (2%) (relative) higher than in the remaining material of the evaporation boat.
Preferably, a layer of the conductive component of the ceramic material is located on the surface of the evaporation boat at which the evaporation of the metal occurs.
The layer of the conductive component should preferably be in electrical contact with the remaining evaporator boat material. As a result, this layer becomes self-conducting and, owing to the low resistivity of the material, becomes hotter than the remaining evaporator boat material. This again leads to an improvement in the wettability of the surface of the evaporator boat.
The concentration of the conductive component at the surface of the evaporation boat is preferably achieved starting from an evaporation boat known from the prior art by means of one of the three methods described below:
1) The surface of the evaporation boat from which the evaporation of the metal is to take place in normal operation is heated by means of a high-energy beam so that the nonconductive components such as BN with a melting point of 2300xc2x0 C. and AlN having a melting point of 2300xc2x0 C. evaporate and at the same time the conductive component such as TiB2 having a melting point of 2900xc2x0 C. is only melted. The energy content of the high energy beam is therefore preferably selected such that it heats the surface of the evaporation boat to more than 2900xc2x0 C. but not less than 2700xc2x0 C. This results, after cooling, in a layer enriched in the conductive ceramic component such as TiB2, on the surface of the of the evaporation boat. More brief heating gives a layer which is less enriched in the conductive ceramic component on the surface of the evaporation boat.
2. Powder comprising the conductive ceramic component can be applied to the surface of the evaporation boat and welded on by means of a high-energy beam to form an electrically conductive layer of the conductive ceramic material. This can be achieved such as through using TiB2 as a conductive ceramic component, and using a method analogous to a TiB2 powder coating process known per se.
3. Powder comprising the conductive ceramic component can be processed with an organic or inorganic binder to form a paste and the paste applied to the surface of the evaporation boat. The binder is selected so that it evaporates during the heating of the evaporation boat.
Thus, when the evaporation boat is heated, the binder evaporates and the desired electrically conductive layer which can subsequently be wetted by aluminum is formed. The binder used can be, for example, glycerol. This layer can be additionally treated by means of a high-energy beam as described in the second process to obtain better contact between the electrically conductive layer and the remaining evaporator material.
As a rule, the electrically conductive component of the ceramic material is TiB2. For this reason, TiB2-containing powder is preferably used as a powder comprising the conductive ceramic component.
The high energy beam can be in the form of a laser beam. This laser can be, for example, a gas, solid state, or semiconductor laser.
The heating of the surface of the ceramic evaporation boat by means of a high-energy beam is preferably carried out under inert gas conditions. Examples of inert gases are helium and argon.
The evaporation boats of the present invention have the following advantages over known evaporation boats:
1) From the start of use, these evaporation boats have good uniform wetting which leads to a constant and uniform (in space) evaporation rate without scattering.
2) They run in a steady manner from the beginning and power does not have to be continually adjusted.
3) the scattering in the resistance of the evaporation boats which results from the scarcely avoidable scatter in the resistance of the sintered body from which the evaporation boats are produced does not have the adverse effect of the higher resistance evaporation boats no longer being wettable and the vapor deposition units therefore have to be stopped.
The following examples illustrate the invention.